In an oil pump mechanism of a conventional portable working machine, a pump shaft housed in a pump case is biased by an elastic body, is prevented from coming off with a fixing pin or the like, and moves forward and backward axially to expand and compress a volume in a pump chamber, thereby sucking and discharging oil. In conjunction with the rotation of the pump shaft, the pump chamber alternately communicates with a suction passage or a discharge passage. A sloped portion is formed on an end face of the pump shaft on a fixing pin side. Oil is sucked into the pump chamber through the suction passage when the pump chamber is expanded, and is discharged from the pump chamber through the discharge passage when the pump chamber is compressed. In order to enhance airtightness between the pump chamber and a gear chamber and airtightness between the pump chamber and the discharge passage, the pump case is manufactured so as to increase the machining accuracy of a portion in which the pump shaft is housed, and the machining accuracy of the pump shaft is also increased to improve the overall airtightness, thereby achieving the stable pumping performance.
Depending on a type of an oil pump, a pump case and a pump shaft are molded from synthetic resin and are assembled and used as it is without mechanical machining. In such a case, when either of a suction port or a discharge port of the pump chamber is closed, a gap between the pump case and the pump shaft is larger than a gap between the pump case and the pump shaft manufactured by mechanical machining. Therefore, a certain measure for reducing the gap is necessary. In Japanese Examined Utility Model Application Publication No. 5-26316 (Patent Document 1), a pushing device which biases a pump shaft toward an axial direction of a closed passage relative to an opening portion of either one of a suction port and a discharge port is provided to improve the sealability of a closed portion, thereby improving the pumping performance.